1. Field of the Invention
The present invention relates to silver halide photographic emulsions having improved sensitivity and to a method for increasing the sensitivity of silver halide photographic emulsions. More particularly, it relates to a silver halide photographic emulsion and a method for increasing the sensitivity of a silver halide photographic emulsion by sulfur-sensitizing it and then incorporating a heterocyclic mercapto compound therein.
2. Description of the Prior Art
In the art of preparing photographic emulsions, it has been desired to increase the sensitivity of photographic emulsions without such being accompanied by fog.
A nitrogen-containing heterocyclic compound having a mercapto group is a known antifoggant and has been employed for the purpose of reducing fog. However, it is known that this compound also aften reduces photographic sensitivity. For example, see C. E. K. Mees & T. H. James "The Theory of the Photographic Processes (3rd. Edition)" pages 344 to 346.
Despite the above prior art belief, the inventor has found that the sensitivity of a sulfur-sensitized silver halide photographic emulsion can be increased by adding a particular nitrogen-containing heterocyclic compound having a mercapto group (which has been generally known as an antifoggant) to a sulfur-sensitized silver halide photographic emulsion containing at least one of silver bromide and silver chloride and silver halide particles of a cubic crystal form. This discovery is unexpected because it has been believed in the art that many nitrogen-containing compounds which are well-known as antifoggants reduce photographic sensitivity while inhibiting fog.